I think the primary reason is branding. The iPhone is iconic, and the springboard UI, with the brightly-colored app icons, is a big part of that. On the original iPhone, there were no 3rd party apps, there was no springboard rearrangement, there was no springboard wallpaper - ALL (non-jailbroken) iPhones looked exactly the same. In addition to keeping things as simple as possible, I really think the idea was to to make the iPhone (UI and all) an icon, just like the iPod was before it. They've loosened up a bit since then, by necessity due to the explosion of the app store, but in general I think this idea is still persistent.
Secondarily, there are performance concerns like Battery life and lag that are always side-effects of customizations, live-wallpapers, and widgets.